Field of the Invention
1. The present invention relates generally to ecologically balanced treatment systems for treating flowing water in order to reduce the quantity of undesirable chemicals found in the water. More specifically, the invention relates to an ecologically balanced treatment system in which flowing water is treated with periphyton in a primary stage and in which algae-grazing fish feed upon the periphyton in a secondary stage. Fish grazing optimizes periphyton growth and incorporates nutrients or toxicants from the periphyton into forms removable from the system.
2. Description of the Prior Art
Freshwater periphyton are organisms whose ecological niche lies within freshwater ecosystems or communities, such as rivers and lakes. Periphyton attach or grow upon surfaces within the water, for example, stems and leaves of rooted and floating plants and rocks. Periphyton communities contain a variety of organisms, including algae, bacteria, viruses and protozoans.
Periphyton algae reside at the bottom of the food chain in freshwater ecosystems. In a food chain, plants such as algae convert the energy of the sun into chemical food energy during photosynthesis. The food energy from the plants passes through the food chain by means of a series of organisms and steps of eating and being eaten. For example in a stream, algae convert sunlight and nutrients to plant materials. Herbivorous fish eat the algae. Carnivorous fish eat the herbivorous fish. Humans catch and eat the carnivorous fish.
Unless broken down, toxicants or toxic substances in the food chain are transferred with every step of consumption. Toxicants often enter the food chain at the lower levels, for instance, the periphyton level. For example, toxicants in the water may attach to algae. When the algae is eaten, the consumer eats the toxicant, too.
Toxicants that are not broken down concentrate in the tissues of the consumer with each transfer step. The effect is cumulative. Low, sometimes undetectable, levels of toxicants in the water accumulate in the consumer's tissues until reaching harmful levels. Harmful levels may be sublethal, yet over time, they contribute to the morbidity and mortality of the organism For example, polychlorinated biphenyl (PCB) compounds are highly toxic chemicals that are not readily degraded in the environment. PCB manufacture was discontinued in 1976 in the United States. PCBs, however, are still found in low concentrations in water communities. PCBs pass through the food chain until consumed by game fish. Game fish caught in these water communities frequently have high concentrations of PCBs in their tissue. Humans who eat the game fish also eat the PCBs. Since humans cannot break down PCBs, the PCBs accumulate in the human body during a lifetime. Measurable levels of PCBs are even found in the breast milk of nursing mothers, passing the PCBs to their offspring.
Water quality deteriorates in freshwater ecosystems with elevated levels of nutrients such as nitrogen and phosphorus. Materials rich in nitrogen and phosphorus, such as fertilizer and manure from agricultural run-off, pollute freshwater ecosystems. Excess nitrogen and phosphorus cause excessive phytoplankton growth or "blooms" in freshwater. For example, nitrogen and phosphorus pollution promotes the increase in cyanobacteria or blue-green algae. Cyanobacteria are a bane to potable water production Cyanobacteria add bad tasting and sometimes toxic excretions or decay products to the water. Phytoplankton blooms, especially those with cyanobacteria, cause a decline in water clarity and palatability. Water drawn from sources with phytoplankton blooms is cloudy, bad tasting and smelly.
In water, nitrogen removal occurs either through the food chain or loss to the atmosphere. Phosphorus removal, in contrast, can only be through the food chain. To withdraw phosphorus from water, organisms incorporate or adsorb phosphorus and transfer it via the food chain. Harvesting the organisms from the water eliminates the phosphorus.
In the freshwater community, periphyton acts as a nutrient sink by taking up both nitrogen and phosphorus, thereby decreasing the nutrients remaining in the water. Periphyton adjusts its population growth and nutrient uptake rates depending on the available nutrient levels. The removal rate of nutrients can be high. For instance, phosphorus removal rates can reach 160 mg P/M.sup.2 /d, while the rate of nitrogen removal can reach 1900 mg N/M.sup.2 /d.
In spite of these recognized beneficial effects, a limiting factor in the use of periphyton to treat flowing water systems lies in the fact that periphyton's role as a nutrient sink is limited, to some extent, to early colonization and growth phases. This is due to the fact that optimal nutrient removal by periphyton depends upon maintaining maximal or rapid growth. If periphyton growth becomes too thick, an interior anaerobic layer forms and maximal growth ends. At this stage, the periphyton actually begins to release some nutrients back into the water. Further, the periphyton slough off and wash downstream. These results are counterproductive for the stated objectives of the present invention.
Another problem addressed by the method of the present invention lies in the fact that flowing water communities differ dramatically from the communities of ponds, lakes and lagoons. Traditional water treatment and sewage treatment facilities rely upon one or more such "ponds" as an integral part of the treatment process. Such systems are not well suited for handling large volumes of flowing water. For example, the water current is more of a factor in the maintenance and growth in a flowing water community than in a pond. Periphyton adjusts to current velocity. Periphyton population growth increases with faster current velocity Larger periphyton populations remove more nutrients from the water, a beneficial effect of the increased current velocity. Oxygen is also more available in flowing water communities than in ponds. Both the greater amount of water surface area exposed to the air and the constant water movement increase the amount of available oxygen in the flowing water community. These benefits are not realized or efficiently utilized by traditional water treatment or sewage treatment processes.
Fish play an important role in the freshwater food chain and the transfer of nutrients such as nitrogen and phosphorus. Fish eat organisms from the lower level of the food chain. Fish incorporate the consumed nutrients into fish tissue or pass nutrients through their intestinal tract as incompletely digested algae and bacteria.
Effective water purification has been a problem over the years. The need for water purification is urgent. For example, toxicants work their way through natural food chains, becoming dangerously concentrated in fish or birds. Lakes and rivers suffocate from heavy phytoplankton blooms. Chemicals found within a river poison plants and animals that depend on the water for survival, both in the river and along its edge. The present invention provides a simple and ecologically balanced method for treating large volumes of flowing water to provide cleaner water while removing undesirable chemicals found in the untreated, flowing water.
Other problems addressed by the method of the present invention include presently available sewage treatment processes. Present sewage treatment facilities are designed to remove nutrients from water, not toxicants. Present sewage treatment usually involves treating the sewage in holding ponds or lagoons over a period of time to remove nutrients from the water. Toxicants pass into and through the sewage treatment facility. By the end of treatment when the water is released to the environment, the toxicants may be at harmful levels.
Contemporary sewage treatment facilities often inadequately handle large amounts of water that enter the system quickly. To prevent flooding during heavy rains, sewage treatment facilities often dump raw sewage into the channels, rivers, lakes or oceans thereby polluting the water. The current also carries the raw sewage to other locations. This pollution jeopardizes drinking water supplies and contaminates recreational water areas.
In some locations, water treatment is nonexistent or substandard. People in these areas drink whatever water is available. The available water is often contaminated. Treating water using standard engineering practices is complicated and expensive. Small municipalities or water districts have shrinking financial resources. These governments need inexpensive water treatment. The method of the present invention provides a simple and inexpensive ecological process for treating large volumes of flowing water in order to produce cleaner water for municipalities or water districts.